The present invention relates to a pneumatic tire for motorcycles, in which running stability under low pressure service conditions is improved.
In general, when running off the road, motorcycle tires for use both on the road and off the road are inflated to a lower air pressure than the normal air pressure therefor, for example, 0.5 to 0.25 kgf/sq.cm (which is about 10 to 20% of the maximum air pressure), whereby the stiffness of the tread portion is reduced, and contact of the tread face with the ground is increased, and as a result a large road gripping force is obtained.
However, in a conventional motorcycle tire, by decreasing the air pressure, the stiffness of the tire is decreased in the sidewall portions as well as the tread portion, and as a result running stability is lost under low pressure service conditions.
In such a conventional motorcycle tire, as shown in FIG. 2, the carcass has a round profile under a low pressure condition, which profile is almost the same shape as that for the ordinary tires used on the road at a higher pressure (normal pressure). Therefore, when loaded, the sidewall portions m are deformed such that the whole of the sidewall portion m is curved outwards of the tire like an arc. However, the deformation is liable to occur unstably and alternately on the sidewall portions, which greatly deteriorates running stability. Further, as the deformation of the sidewall portions flatten the tread portion n, suppleness of the tread portion is apt to be lost.
Furthermore, the deformation occurs all over the tire as mentioned above, and accordingly it takes a long time to recover from the deformed state to its usual state since the air pressure is low, which also deteriorates the running stability.
It is therefore, an object of the present invention to provide a low pressure motorcycle tire in which running stability is improved by arranging the tire shape more particularly by arranging the carcass profile under a not fully inflated condition.
According to one aspect of the present invention, a low pressure motorcycle tire has a pair of bead portions, a tread portion, and a pair of sidewall portions, and the tire comprises a bead core disposed in each bead portion, and a carcass extending between the bead portions and secured to the bead cores, and
the ratio (A/B) of the tire section width (A) to the bead width (B) when the tire is mounted on a regular rim and inflated to 15% of the maximum pressure is not less than 1.90 and not more than 2.20, wherein PA1 the tire section width (A) is measured at the same radial height as the carcass maximum section width point (p) in the tire sidewall portions at which the section width of the carcass is maximum.
Therefore, axially outward swelling of the sidewall portion is increased more than the conventional tire sidewall, and the sidewall shape under low pressure conditions becomes similar to it loaded shape, that is, the difference between the loaded shape and free shape becomes small, and deformation occurs stably on both the sidewall portions, and the amount of deformation is less. As a result, running stability is improved while the tire stiffness is maintained in both the vertical direction and lateral direction.
However, if the A/B ratio is more than 2.20, the amount of swelling is excessive, and the vertical stiffness has a tendency to decrease, and further, tire performances at the normal pressure such as wear resistance and cut resistance of the tread are sacrificed. On the other hand, when the A/B ratio is less than 1.90, the lateral stiffness is decreased and running stability is lost.
Further, the carcass profile is preferably arranged as follows:
a middle portion (C) of the carcass profile defined between a point (W1) and a point (W2) is formed by an arc (Sc) having a curvature of a single radius (Rc) and having a center (o1) inside the tire;
the point (W1) is located radially outside the above-mentioned carcass maximum section width point (p);
the point (W2) is located radially inside the carcass maximum section width point (p);
the radial distance (L2) between the points (W2) and (p) is more than 1.0 times and not more than 1.2 times the radial distance (L1) between the points (W1) and (p);
the radial distance (L) between the points (W1) and (W2) is not less than 0.4 times and not more than 0.6 times the radial height (H) of the carcass measured from the bead base (b) to a point (a) on the tire equator; and
the radial distance (L3) measured from the bead base (b) to the above-mentioned radially inner point (W2) is not less than 0.15 and not more than 0.35 times the above-mentioned radial height (H) of the carcass.
Accordingly, the curved middle portions of the carcass induce stable bending deformation in the sidewall portions and restrict the deformation in a narrow region whereby the recovering time is shortened. Further, the middle portions help the increase in the vertical stiffness of the tire under both a low pressure and the normal pressure.
Furthermore, a radially inner portion (U) of the carcass profile defined as extending from the radially inner point (W2) to the bead base region is preferably formed by an arc (Su) having a center (o2) outside the tire and having a curvature of a single radius (Ru) smoothly connected to the above-mentioned arc (Sc) of the middle portion (C) at the point (W2) as a point of inflection therebetween.
Therefore, the inner portion of the carcass forwards the occurrence of the above-mentioned stable deformation of the sidewall and increases the lateral stiffness of the tire.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.